1. Field of the Invention
The present invention relates to an anti-reflective film coated on a surface of an optical element and an optical element having an anti-reflective film, and particularly to an anti-reflective film effective for ultraviolet light with a wavelength of 140 to 210 nm.
2. Description of the Related Art
Generally, as an anti-reflective film in an optical element such as an optical lens, a multilayer film is used which has high-refraction films and low-refraction films stacked in layers. The larger the number of the layers, the better the anti-reflective function of the film. However, when the number of layers of an anti-reflective film is increased in order to improve the anti-reflective function, the whole thickness of the anti-reflective film increases. When the thickness of the anti-reflective film becomes large, the transmittance of the anti-reflective film will lower to degrade the function as an optical element. Particularly, when using an anti-reflective film with a low refractive index, because it is necessary to increase the number of anti-reflective layers to make larger the geometrical film thickness in order to attain a desired optical function, the transmittance will lower to degrade a function as an optical element to vacuum-ultraviolet light.
It has hitherto been known to use a fluoride film for an anti-reflective film to vacuum-ultraviolet light. U.S. Pat. No. 6,472,087 discloses an optical element using a fluoride film for an anti-reflective film, more specifically an anti-reflective film effective for an ultraviolet light of a wavelength of 190 to 250 nm obtained by employing Al2O3 as a high refractive-index layer and AlF3 or MgF2 as a low refractive-index layer and stacking them alternately. Further, WO 00/058761 discloses an anti-reflective film consisting of 5 to 10 layers using a fluoride film.
However, the wavelength of a light source used has become shorter and shorter in recent years, and even an F2 laser of a wavelength of 157 nm has begun to be used. For this reason, an anti-reflective film effective in shorter wavelengths has been required. U.S. Pat. No. 6,472,087 mentioned above discloses an anti-reflective film effective for a wavelength within a range of 190 to 250 nm. However, the reflectance thereof is as high as 0.3% or more at a wavelength of 169 nm or less and is as high as 1% or more at a wavelength of 163 nm or less. In addition, an anti-reflective film having six layers disclosed in U.S. Pat. No. 6,472,087 above has a problem that the reflectance cannot be made low for a light. incident at such a large angle as 30 degrees or.more.
In addition, of anti-reflective films having 5 to 10 layers disclosed in WO 00/058761 above, an anti-reflective film having 6 layers also has a problem that the reflectance cannot be made low for a light incident at such a large angle as 30 degrees or more as is the case with U.S. Pat. No. 6,472,087. Moreover, for an anti-reflective film having 10 layers, the whole thickness is as very large as 3.320 λ0. Therefore, it is believed that the transmittance to ultraviolet light is considerably low.